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A centrifugal habitat for reduced gravity environments
Abstract
The human exploration of space occasions travel, work and residence in environments having different coefficients of gravity. Increasing attention has been given over its history to the adverse physiological effects of prolonged exposure to reduced or zero gravity. Muscle atrophy and bone demineralisation are the foremost cumulative symptoms, the latter of which is believed to be irreversible under present medical technology. Rotating space stations and spacecraft can simulate gravity in orbit or during inertial transit but this provision for reduced gravity environments such as exist on Luna, Mars and the outer moons has not been adequately addressed. This paper describes a class of centrifugal habitats for reduced gravity environments which promise to provide incremental levels of simulated gravity in accommodations which are comfortable, spacious and readily accessible to outposts and colonies on planetary surfaces. The members of this class share a common geometric basis but exhibit infinite variety both in scale and in the details of their implementation.
The human species is rapidly expanding. Barring global catastrophe and unnatural constraints, it will, in a brief time on the scale of natural history, fill the uttermost reaches of the solar system. The beachhead established by President Kennedy's lunar program will lead to lunar, Martian and free space settlements in the next century. In a single generation of those who call them home, the constant 9.81 m.s-2 pull of Earth's gravity, which has influenced the evolution and development of terrestrial life forms for billions of years, will fade from common experience. Miogravity syndrome, a prognosticated complex arising in reduced gravity environments such as the surfaces of the Moon and Mars and principally encompassing muscle atrophy, cardiovascular deconditioning and bone demineralization, stands to replace physics and rocketry as the fundamental challenge of interplanetary astronautics. Mirroring our past few million years of changing climate and resources, the mobility of humans between diverse gravitational environments on the high frontier will critically depend on our ability to adapt. Tomorrow, as ever, a mushrooming penchant for toolmaking will spearhead the human career.
Design concepts are described for self-replicating systems and molecular manufacturing techniques for applications to space exploration, and a generic 'assembler' concept is proposed. The von Neumann (1966) architecture for a self-replicating system is described and extended to Drexler's architecture for an assembler. The assembler is based on a molecular computer coupled with a molecular constructor that has molecular positional capability and 'tip chemistry' capabilities. Specifications for an assembler are given that permit the definition of its capabilities, functional environment, and the control of its progress. Critical elements of the assembler design are the tip chemistry and maintenance of the suitable internal environment. Applications for the assemblers include molecular manufacturing of a range of products and structures for use in space exploration.
From the perspective of energy expenditure, ice is the most economical water resource to target for exploration on Mars. Theoretical stability criteria indicate the planetary-scale potential for ground ice poleward of about 40° latitude. Geologic indicators can constrain the exploration. Particularly useful in this regard are fluidized ejecta blankets, periglacial features, and relict glacial landforms. The relationship of such geomorphological indicators to the modern water resources is dictated by the processes responsible for water cycling in the Martian past and the extension of those processes to the present. The geomorphological evidence indicates extensive water cycling in the geologic past. Exploration strategies can develop around the resource potential of hydrated minerals, hydrothermal systems, and ground ice based on an evolving practical experience as resources are discovered.
The spacecraft flybys of Comet Halley in 1986 confirmed Whipple's icy conglomerate hypothesis for cometary nuclei and showed that comets are far richer in volatiles than any other class of solar system bodies. Water is the most abundant volatile, comprising roughly 80 percent of the gas flowing out from the nucleus. Carbon monoxide is next with a content of 15 percent relative to water, though with approximately half of that coming from an extended source in the cometary coma, i.e., hydrocarbon dust grains. The detection of large numbers of hydrocarbon CHON grains was one of the more significant discoveries of the Halley flybys, as was the ground-based observation that CN occurs in jets, again indicating an extended source. Evidence was also found for more complex hydrocarbons. Estimates of the total dust-to-gas ratio for Halley range as high as 2:1, indicating that a substantial fraction of the volatile material may be tied up in solid hydrocarbons rather than ices. The role of clathrates in trapping more volatile ices is not yet understood. If Halley can be taken to be representative of all short-period comets, then the short-period comets may provide a significant source of volatiles in near-earth space. This resource is more difficult to reach dynamically than the near-earth asteriods, but the high volatile content may justify the additional effort necessary. In addition, there is considerable evidence that at least some fraction of the near-earth asteriods are extinct cometary nuclei which have evolved into asteroid orbits, and which may contain significant volatiles buried beneath an insulating lag-deposit crust of nonvolatiles. Knowledge of comets will be greatly enhanced in the near future by the Comet Rendezvous Flyby mission now under development by NASA, and by the proposed Rosetta mission.
This study attempted to determine which visual scene cues are most effective in overcoming the somatogravic illusion (SGI), a form of spatial disorientation that occurs when a shift in the resultant gravitoinertial force vector created by a sustained linear acceleration is misinterpreted as a change in pitch or bank attitude. Nine subjects were exposed to a gravitoinertial force shift of -30 degrees in the pitch plane, both with their eyes closed and while viewing computer-generated visual scenes through a wide field-of-view head-mounted display. The scenes depicted acceleration over a shoreline by means of horizon, texture, perspective, and color cues that were presented both in isolation and in various combinations. None of the scenes significantly reduced the magnitude of the SGI relative to the eyes-closed (baseline) pitch illusion, even though the textured scenes produced some linear vection. It remains to be established whether low-cost head-mounted visual displays can reliably reduce the magnitude of the SGI and other spatially disorienting illusions.
A longitudinal study evaluated the susceptibility to motion sickness in initially unfit subjects before and after an endurance training program. Motion stimulation was provided by the Precision Angular Mover, in which the subject was tumbled head over heels about an Earth-horizontal axis at 20 cycles per minute in darkness. Maximal aerobic power and the blood lactate response to submaximal exercise were evaluated with cycle ergometry. The training program caused significant improvements in VO2max and endurance capacity, and a significant decrease in percent body fat. There was a significant (p less than 0.0125) increase in motion sickness susceptibility after the physical training, suggesting that increased physical fitness caused increased susceptibility to motion sickness in some individuals.
Molecular nanotechnology involves the ability to manufacture objects to precise atomic specifications. A central postulate is that any structure that can be specified and that does not violate physical law can be built. Three pathways to molecular nanotechnology are proximate probe technology (the use of improvements of the scanning tunneling microscope, STM), biotechnology, and supramolecular chemistry. Combinations of these technologies appear particularly powerful. The biotechnological approach should make it possible to use in vitro translation systems to manufacture polymers containing at least 10 times as many different artificial monomers as there are natural amino acids. These polymers could further adsorb various other molecular devices, and the use of STMs should enable the complexes to be arranged into sophisticated machines, including molecular computers. The implications include pocket superautomated analyzers and the ability to base medical therapy on the biochemical individuality of specific patients.
This paper introduces a new repulsive magnetic levitation approach
using permanent magnets and air-core electromagnets as primary actuating
components. The permanent magnets, which are attached to the bottom of a
carrier, are repulsively levitated above and by oblong shaped
electromagnets, which constitute one part of the guide tracks. Due to
the lateral unstable nature of repulsive levitation, the stability of
the levitated permanent magnets is regulated by another part of the
guide tracks, electromagnetic stabilizers, which are strands of straight
wires running through the entire length of the guide tracks above the
levitation coils. A state feedback controller with integral compensator
is designed for the stability control. The entire levitation system is
divided into three subsystems: levitation, stabilization and propulsion.
All the control works with respect to each subsystem are executed
extrinsic to the carrier, i.e., there is no electrical circuit on board
the carrier. The feasibility of this new levitation approach is
confirmed successfully by a small scale demonstration system designed to
transport a single silicon wafer in a microelectronic fabrication line.
The wafer carrier is stably levitated 1.5 mm above the track surface
with lateral air-gaps of 0.5 mm. Its maximum velocity is 22.2 cm/sec and
the traveling range is 1 m
The meteorites that fall on Earth are necessarily samples of the
near-Earth population of small solar system bodies, and hence are a very
useful guide to the nature of the materials available in near-Earth
space. The authors review the compositions of the most abundant classes
of noncarbonaceous meteorites that fall on Earth for the purpose of
identifying economically attractive uses of near-Earth asteroid
materials.
The moon is nearly devoid of essential biogenic resources such as water, hydrocarbons, and nitrogen. Lunar bases must have a ready supply of these vital resources since they are easily lost to the vacuum of space. Also, wet chemical processes dominate the chemical industries. Extraterrestrial sources of these materials must be found to provide for life support, construction, and manufacturing. If Phobos and Deimos have carbonaceous chondritic compositions, they are ideal targets for extraterrestrial exploitation. They may contain biogenic resources such as water, hydrocarbons, and nitrogen, as well as easily recoverable structural materials.
There exist frequent, low DeltaV mission opportunities to many bodies in the complete near-Earth asteroid (NEA) population: about 250 of these asteroids larger than 1 km diameter are easier to reach than the Moon's surface. Opportunities for fast missions of less than one year duration that require less DeltaV than a round-trip lunar mission occur about 9 to 10 times each year on average to a near-Earth asteroid larger than 0.5 km in diameter. However, less than 10% of the approximately 1,700 NEAs larger than 1 km have been discovered through mid-1991. Hence, a significant increase in the discovery rate of NEAs is needed to realize the mission potential in the NEA population.
The history of interest in and development of the space station concept
is given. It is pointed out that the decision to base manned lunar
missions on the 'lunar orbit rendezvous method', as opposed to Earth
orbit staging, slowed development of space station proposals in the
1960s. Planned use of Skylab as a space station with Space Shuttle
resupply failed when skylab returned to Earth in 1979 before the first
Space Shuttle could be launched. The reluctance of Johnson Space Center
officials to involve astronauts in the kind of lengthy extra-vehicular
activity (EVA) required for space station construction was, and
continues to be, a major sticking point. After a symposium in 1983 aired
the EVA isues extensively, a proposal was presented for a baseline
configuration space station program, and it was accepted on December 1,
1983. A series of reconsiderations of the baseline configuration led to
the space station Freedom concept, which would cost significatly more
than original estimates. This led to deterioration in interest in the
program, leading to near death of the program by 1992. A major
reconsideration study mandated by the Clinton Administration has
resulted in abandonment of the Freedom concept and a return to a
reduced-scope version of the project. Congress approved the program by a
narrow margin on June 21, 1993.
The results of an analysis of molecular manufacturing systems are
reviewed to study the development of space hardware with attention given
to radiation damage. The general principles are discussed for using
high-frequency mechanisms to build macroscopic structures from molecular
components. The efficient development of highly effective products is
predicted due to: (1) the miniaturization of computer elements; (2) the
use of high-density power conversion; (3) the mechanosynthesis of
carbon-rich materials; and (4) the potentially high reliability of the
high-frequency manufacturing techniques. Also expected are highly
redundant systems with limited defects manufactured in a
high-productivity environment for use in earth-to-orbit and
interplanetary transportation. Radiation damage is a critical limiting
factor in the design and use of molecular technologies and should be
carefully monitored.
Advanced principles of integrated circuit fabrication can be applied for three-dimensional microstructures. This technology based on photofabrication is called micromachining. The micromachining makes it possible to fabricate micromachines which have sensors, actuators and electronic circuits on a silicon wafer. Microflow control systems and integrated mechanical sensors were fabricated. Micromachining and micromachines of silicon are reviewed.
Magnetic levitation and MHD propulsion are now attracting attention in several countries. Different superconducting MagLev and MHD systems will be described concentrating on, above all, the electromagnetic aspect. Some programmes occurring throughout the world will be described. Magnetic levitated trains could be the new high speed transportation system for the 21st century. Intensive studies involving MagLev trains using superconductivity have been carried out in Japan since 1970. The construction of a 43 km long track is to be the next step. In 1991 a six year programme was launched in the United States to evaluate the performances of MagLev systems for transportation. The MHD (MagnetoHydroDynamic) offers some interesting advantages (efficiency, stealth characteristics, ...) for naval propulsion and increasing attention is being paid towards it nowadays. Japan is also up at the top with the tests of Yamato I, a 260 ton MHD propulsed ship.
This paper examines the possibility of mapping the surface composition of the moon from an orbiting spin-stabilized spacecraft, using gamma ray spectroscopy and a cooled germanium solid-state device as a detector. A design for accommodating the germanium detector gamma ray spectrometer was devised, and the detection sensitivity was applied to typical lunar-rock compositions. For sets comprising nine highland and 16 mare types, the most useful elements were found to be Mg, Al, K, Ti, Fe, U, and Th. An analysis of the expected instrument response to the gamma ray and neutron fluxes of water ice indicated that a neutron mode added to the spectrometer will be more sensitive than the gamma ray mode to the possible presence of polar ice. It was calculated that, with a pair of selected neutron absorbers and a model which provides that 2.5 percent of the area above 75-deg latitude is occupied by trapping sites, the instrument will provide a 1-yr mission detection limit of 0.056 percent H2O by weight for each polar region.
Seasickness is the most prevalent form of motion sickness and is an operational problem during Space Shuttle Solid-fueled Rocket Booster (SRB) retrieval. Phenytoin has been shown to protect against motion sickness induced by Coriolis stress. We exposed SRB recovery personnel to off-vertical rotation and sea motion after phenytoin or placebo. Phenytoin blood levels of at least 9 micrograms/ml were protective against motion sickness at sea. No change in susceptibility to nitrogen narcosis was seen in divers in chamber tests at 460 KPa. Phenytoin was used during the performance of critical and hazardous tasks during training and actual SRB recovery operations. Phenytoin is an effective operational countermeasure for motion sickness for selected SRB crew members.
Recent analysis analytical electron microscope study of lunar soils showed that the approximately 60-nm-wide amorphous rims surrounding many lunar soils grains exhibit distinct compositional differences from their hosts. On average, the amorphous rim compositions reflect the local bulk soil composition with the exceptions of Si and S, which are enriched relative to the bulk soil. These chemical trends led us to propose that the amorphous rims were in fact deposits of impact-generated vapors produced during regolith gardening, a hypothesis that runs contrary to the generally accepted view that the rims are produced through amorphization of the outer parts of mineral grains by interaction with the solar wind. Analytical data are reported for amorphous rims on individual minerals in lunar soils in order to show that the magnitude of the chemical differences between rim and host are so great that they require a major addition of foreign elements to the grain surfaces. The average composition of amorphous rims is listed as a function of host mineralogy as determined in microtone thin sections using energy-dispersive X-ray spectrometry in the transmission electron microscope. As the host mineral becomes chemically more complex, the chemical differences are not as clear. The average rim compositions are remarkably similar and are independent of the host grain mineralogy. Whether there are 'sputtering' or radiation effects superimposed on the vapor-deposited material can be debated. We do not explicitly exclude the effects of radiation damage as a contributing factor to the formation of amorphous rims; we are merely emphasizing the major role played by condensed vapors in the formation of amorphous rims on lunar soil grains.
Early attempts to develop an animal model for anti-motion sickness drugs, using dogs and cats; were unsuccessful. Dogs did not show a beneficial effect of scopolamine (probably the best single anti-motion sickness drug for humans thus far) and the findings in cats were not definitive. The authors have developed an animal model using the squirrel monkey (Saimiri sciureus) of the Bolivian phenotype. Unrestrained monkeys in a small lucite cage were tested in an apparatus that induces motion sickness by combining vertical oscillation and horizontal rotation in a visually unrestricted laboratory environment. Signs of motion sickness were scored using a rating scale. Ten susceptible monkeys (weighing 800-1000 g) were given a total of five tests each, to establish the baseline susceptibility level. Based on the anticholinergic activity of scopolamine, the sensitivity of squirrel monkey to scopolamine was investigated, and the appropriate dose of scopolamine for this species was determined. Then various anti-motion sickness preparations were administered in subsequent tests: 100 ug scopolamine per monkey; 140 ug dexedrine; 50 ug scopolamine plus 70 ug dexedrine; 100 ug scopolamine plus 140 ug dexedrine; 3 mg promethazine; 3 mg promethazine plus 3 mg ephedrine. All these preparations were significantly effective in preventing motion sickness in the monkeys. Ephedrine, by itself, which is marginally effective in humans, was ineffective in the monkeys at the doses tried (0.3-6.0 mg). The squirrel monkey appears to be a good animal model for antimotion sickness drugs. Peripherally acting antihistamines such as astemizole and terfenadine were found to be ineffective, whereas flunarizine, and an arginine vasopressin V1 antagonist, showed significant activity in preventing motion sickness.
The blood levels of histamine and 5-hydroxytryptamine (5-HT) in 10 subjects, with or without administration of the transdermal therapeutic system of scopolamine (TTS-S), were measured following motion sickness (MS) induced by Coriolis stimulation. Histamine and 5-HT were assayed using the fluorometric method. The results demonstrated that the blood levels of histamine increased significantly following MS and were even higher in the subjects using TTS-S, but we found neither significant changes in the blood levels of 5-HT following MS nor any effect of TTS-S on it. The results suggest that histamine contributes to the development of MS, and scopolamine may exert its anti-MS action by affecting the histaminergic system as well as the acetylcholinergic system; there may not be a definite relation between 5-HT and the development of MS.
Fundamental approaches in selection of new agents for evaluation in prevention of space/motion sickness (SMS) are reviewed. The discussion centers on drugs under investigation at the Johnson Space Center. Methodology that employs the rotating chair for measuring SMS symptomatology and susceptibility is described. The most obvious approach to the development of new agents relies on selection of agents from drug classes that possess pharmacologic properties of established anti-motion sickness agents. A second approach selects drugs that are used to prevent emesis caused by means other than exposure to motion. The third approach relies on basic research that characterizes individual differences in susceptibility. The hypothesis is: detection of individual differences leads to identification of specific drugs, which target physiologic systems that show individual differences. These physiologic systems are targets for therapy and may play a role in the etiology of SMS. Two drugs that reduce susceptibility to SMS include dexamethasone and d(CH2)5Tyr(Me)AVP, a vasopressin (AVP)V1 antagonist. The latter peptide has demonstrated complete blockade of emesis and other significant symptoms in squirrel monkeys. These studies were predicated on observations that subjects who were more resistant to SMS had higher plasma AVP after severe nausea than subjects with lower resistances. Investigations are underway to test a 0.5-mg intravenous dose in humans. Kappa opioid agonists inhibit AVP release and offer new therapeutic possibilities and advantages over AVP peptides. This review details the experimental data collected on AVP and adrenocorticotropin. The literature supports interrelated roles for AVP and opioid peptides in SMS. Experimental testing of kappa agonists is warranted because specific opioid agonists act at neuroanatomical sites causing nausea and vomiting. It is argued opioid receptors in the chemoreceptor trigger zone and vomiting center stimulate and inhibit the emetic response, respectively. The evidence suggests kappa and/or mu receptors at VC are involved in inhibition of emesis, whereas delta opioid receptors at CTZ are involved in stimulation of emesis.
Because maintenance antivertiginous treatment with commonly used drugs is only moderately effective, there is still need for new therapeutic concepts in the therapy of vestibular vertigo. The cerebral calcium antagonist flunarizine (Sibelium) revealed positive vestibular effects in experimental animal studies and in healthy volunteers. Clinical trials versus placebo and reference drugs proved flunarizine to be effective in the treatment of vestibular disorders. Somnolence, weight gain, and, in rare cases, extrapyramidal symptoms and depression were discussed as side effects. Further studies on flunarizine's mechanism of action are needed to elucidate whether its clinical effects are indeed due to calcium-entry blockade.
Two principal reasons stand in the way of the space biomedical research: (1) astronauts were and most of them remain reluctant to be involved in medical investigations, claiming that no operational objectives were ever missed because of the discomforts suffered and (2) many medical researchers concluded too soon that the space related problems were largely related to terrestrial motion sickness. The paper argues for the necessity of the development of a systematic clinical characterization protocol, the use of that protocol on an adequately sized cadre of dedicated physician observer-test subjects from private research sector, and the repeated flights of that cadre to investigate the effects of countermeasures. The application of this protocol should begin by flying pairs from the physician cadre on each of the short Space Shuttle flights taking place and then reflying the cadre for longer duration, and, eventually, on the Space Station Freedom.
Head and body movements made in microgravity tend to bring on symptoms of motion sickness. Such head movements, relative to comparable ones made on Earth, are accompanied by unusual combinations of semicircular canal and otolith activity owing to the unloading of the otoliths in OG. Head movements also bring on symptoms of motion sickness during exposure to visual inversion (or reversal) on Earth because the vestibulo-ocular reflex is rendered anti-compensatory. Here, we present evidence that susceptibility to motion sickness during exposure to visual inversion is decreased in a 0G relative to a 1G force background. This difference in susceptibility appears related to the alteration in otolith function in 0G. Some implications of this finding for the etiology of space motion sickness are described.
The purpose of the work was to study the mechanism responsible for adaptive reorganization of membranes--lipid peroxidation during simulated motion sickness (MS) in animals. It was shown that during MS rats exhibited an elevation of the level of diene conjugates and Schiff foundations in the brain indicating lipid peroxidation activation. A similar increase of phospholipase A2 activity in the brain was noted. In the liver, myocardium and erythrocytes the above changes were insignificant. An antioxidant beta-tocopherol inhibited lipid peroxidation in the rat brain and possessed the vestibule-protective effect in cats that was presumably related to its membrane-stabilizing action.
In a placebo-controlled double-blind crossover pilot study of acute Coriolis-induced motion sickness treatment/prevention in humans employing an anticonvulsant dose of phenytoin, a mean increase in tolerance to motion stress from 4.87 min (S.D. = 5.55) to 46.87 min (S.D. = 32.6) was obtained. This represents a greater than fourfold improvement in efficacy over any currently available single agent and is more than twice as effective as the scopolamine/dexadrine combination. There were none of the usual side effects of blurred vision, dizziness, dry mouth, or sedation.
People who are prone to motion sickness have a directional preponderance of nystagmus to the left. Centrifuging will change this preponderance to the right in most people, and at the same time reduce their tendency towards motion sickness but only with regard to air travel.
We have previously shown that objective measurements of blood flow changes constitute a useful index in assessing susceptibility to nausea produced by cross-coupled angular accelerations (Coriolis stimulation). The findings of this study confirm the value of using the same technique for assessing the degree of nausea produced by pseudo-Coriolis effects.
The manned exploration of the solar system and the surfaces of some of the smaller planets and larger satellites requires that we are able to keep the adverse human physiological response to long term exposure to near zero and greatly reduced gravity environments within acceptable limits consistent with metabolic function. This paper examines the physiological changes associated with microgravity conditions with particular reference to the weightless demineralizatoin of bone (WDB). It is suggested that many of these changes are the result of physical/mechanical processes and are not primarily a medical problem. There are thus two immediately obvious and workable, if relatively costly, solutions to the problem of weightlessness. The provision of a near 1 g field during prolonged space flights, and/or the development of rapid transit spacecraft capable of significant acceleration and short flight times. Although these developments could remove or greatly ameliorate the effects of weightlessness during long-distance space flights there remains a problem relating to the long term colonization of the surfaces of Mars, the Moon, and other small solar system bodies. It is not yet known whether or not there is a critical threshold value of 'g' below which viable human physiological function cannot be sustained. If such a threshold exists permanent colonization may only be possible if the threshold value of 'g' is less than that at the surface of the planet on which we wish to settle.
It is generally agreed that the single factor that most limits human survivability in non-Earth environments is the phenomenon of bone demineralization and the medical problems induced by the subsequent imbalance in the calcium metabolism. Alterations of skeletal properties occur as a result of disturbances in the normal mechanical loading environment of bone. These alterations or "adaptations" obey physical laws, but the precise mathematical relationships remain to be determined. Principles governing unloading and overloading of bone are gaining more attention as a consequence of the planning of manned space stations, moon and Mars bases and spaceflights of long duration. This paper reviews the subject of bone remodeling and presents a mathematical framework which allows for the prediction of skeletal adaptation on Earth and in non-Earth gravity environments by power law relationships.